The controlled radical polymerization (CRP) processes have gained increasing attention because CRP couples the advantages afforded by conventional free radical polymerization (RP) to (co)polymerize a wide range of monomers using various commercially viable processes with the ability to synthesize polymeric materials with predetermined molecular weight (MW), low polydispersity (PDI), controlled composition, site specific functionality, selected chain topology and incorporate bio- or inorganic species into the final product.
The three most studied methods of controlling radical polymerization are nitroxide mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and degenerative transfer with dithioesters via reversible addition-fragmentation chain transfer polymerization (RAFT). Each of these methods relies on establishment of a dynamic equilibrium between a low concentration of active propagating chains and a predominant amount of dormant chains that are unable to propagate or terminate as a means of extending the lifetime of the propagating chains. The low concentration of active species reduces the probability of bimolecular termination reactions, leading to radical polymerization processes that behave as a “living” system. In order to control molecular weight and molecular weight distribution there should be quantitative fast initiation (Ri), at least as fast as propagation(Rp), (Ri<<Rp to Ri˜Rp controlling DPn (^[M]/[I]0) where [M] is the moles of monomer polymerized and [I] is the initial concentration of the added initiator).
However, since CRP processes are radical based polymerization processes some degree of termination reactions are unavoidable. In all radical polymerizations, biradical termination (kt) occurs with a rate which is dependent on the concentration of radicals ([P*]) to the power two, (Rt=kt[P*]2). Therefore, at the same polymerization rate (the same concentration of radicals), essentially the same number of chains would terminate, regardless if a conventional RP or a CRP system had been employed. This ignores to some degree the diffusion effect of macro-radicals since in a RP most chains are terminated by the reaction of a small radical with a growing polymer radical. In the case of SFRP, or ATRP, these initial termination reactions push the equilibrium to the left hand side, (increasing kdeact) as a consequence of forming an excess of dormant species, as a result of the persistent radical effect, [Fischer, H. Chem. Rev. 2001, 101, 3581-3610.] With the net result that in the conventional process, all chains are terminated, whereas in CRP, as a result of the greater number of growing chains, the terminated chains constitute only small fraction of all chains (˜1 to 10%). The remaining species are dormant species, capable of reactivation, functionalization, chain extension to form block copolymers, etc. Thus CRP behaves as a “living” system. [Greszta, D. et. al. Macromolecules 1994, 27, 638.] Additionally, relatively fast initiation, at least as fast as propagation, provides control over molecular weight (DPn=Δ[M]/[I]0; i.e. the degree of polymerization is defined by the ratio of concentrations of the consumed monomer to the introduced initiator) and narrow molecular weight distribution.
As used herein, “polymer” refers to a macromolecule formed by the chemical union of monomers, typically five or more monomers. The term polymer includes homopolymers and copolymers including random copolymers, statistical copolymers, alternating copolymers, gradient copolymers, periodic copolymers, telechelic polymers and polymers of any topology including block copolymers, graft polymers, star polymers, bottle-brush copolymers, comb polymers, branched or hyperbranched polymers, and such polymers tethered to particle surfaces or flat surfaces as well as other polymer structures.
ATRP is the most frequently used CRP technique with a significant commercial potential for many specialty materials including coatings, sealants, adhesives, dispersants but also materials for health and beauty products, electronics and biomedical applications. The most frequently used ATRP procedure is based on a simple reversible halogen atom transfer catalyzed by redox active transition metal compounds, most frequently copper.
ATRP is considered to be one of the most successful controlled/“living” radical processes (CRP) and has been thoroughly described in a series of co-assigned U.S. Patents and Applications, such as U.S. Pat. Nos. 5,763,548; 5,807,937; 5,789,487; 5,945,491; 6,111,022; 6,121,371; 6,124,411; 6,162,882; 6,407,187; 6,512,060; 6,538,091; 6,541,580; 6,624,262; 6,624,263; 6,627,314; 6,759,491; and U.S. patent applications Ser. Nos. 09/534,827; 09/972,056; 10/034,908; 10/269,556; 10/289,545; 10/638,584; 10/860,807; 10/684,137; 10/781,061 and 10/992,249, all of which are herein incorporated by reference. ATRP has also been discussed in numerous publications with Matyjaszewski as co-author and reviewed in several book chapters. [ACS Symp. Ser., 1998, 685; ACS Symp. Ser., 2000; 768; Chem. Rev. 2001, 101, 2921-2990; ACS Symp. Ser., 2003; 854.] Within these publications similar polymerization procedures may be referred to by different names, such as transition metal mediated polymerization or atom transfer polymerization, but the processes are similar and referred to herein as “ATRP”.
ATRP has certain advantages. Many commercially available initiators may be used and various macroinitiators, including wafers, inorganic colloids, glass, paper, and bio-active molecules including proteins, DNA, carbohydrates and many commercial polymers may be simply synthesized. Many polymers produced by ATRP allow facile functionalization or transformation of the end groups by replacing terminal halogens with azides, amines, phosphines and other functionalities via nucleophilic substitution, radical addition or other radical combination reactions. An abundance of (co)polymerizable monomers are available. This allows production of macromolecules with complex topology such as stars, combs and dendrimers, coupled with the ability to control composition and hence functionality in block, gradient, periodic copolymers etc. and even control polymer tacticity. The procedure is a simple procedure which may be carried out in bulk, or in the presence of organic solvents or in water under homogeneous or heterogeneous conditions, in ionic liquids, and in supercritical CO2.
Many commercial plastic materials are made non-flammable by the use of organohalogen type fire retardants. This method, however, poses problems such as toxicity of fire retardants, corrosion of equipment during melt processing, and emission of smoke and toxic fumes during processing and in subsequent fires. Therefore, much attention has been paid to replacing this type of fire retardant with inorganic materials such as aluminum hydroxide (ATH) and magnesium hydroxide (MDH), which are nontoxic and avoid the above-mentioned difficulties. This type of flame retardant is perhaps the most environmentally friendly type of flame retardant since both ATH and MDH release only water vapor during a fire. Released water can block the flame and exclude oxygen by diluting the presence of flammable gases in the contacting atmosphere. In addition, char formed on the surface of the polymer works as a heat insulating barrier so it interrupts the flow of flammable decomposition products. The additives can work alone or in the presence of other intumescent additives. However, ATH begins to dehydrate at about 180° C. and is hence unusable for use in thermoplastics resins, such as polyesters whose processing temperature is at least 200° C. MDH, on the other hand, has the advantage that its decomposition into MgO and H2O starts at a relatively high temperature (300-320° C.), thus allowing it to be melt compounded into plastics for which ATH is not sufficiently thermally stable.
Both ATH and MDH have some drawbacks. To be effective as a flame retardant, high filler loading (60 wt %) is necessary, resulting in a significant loss in mechanical properties, especially in elongation at break and stress whitening in bending deformation. Recently there has been a great demand to develop thermoplastic based resins as cable insulating materials for the cable industry. This is particularly true for automotive cable insulation applications where plasticized PVC and PE/EVA are the main polymers currently used. PVC, although a better fire retardant polymer than polyolefins, is a source of health and environmental problems due to PVC's potential for release of chlorine-containing chemicals.
The state of the current art for polypropylene composites filled with MDH particles has been provided by Hong et. al. [Hong, et.al: Journal of Applied Polymer Science 97: 2311-2318, 2005] In their summary of prior art, it is noted that the morphology, size, dispersion, and applied surface coating on particles influence the mechanical properties of MDH filled PP composites. It was found that the tensile yield strength decreased in proportion to the increase in the incorporated amount of MDH, because there was no adhesion between the filler and the polymer matrix. However, surface coating with sodium stearate was shown to enhance the compatibility with the resin, but did not afford a chemical bond between the filler and matrix. Further, surface coating with stearic acid led to the reduction in tensile yield strength of the composite compared to composites containing pure MDH, due to the lower thermodynamic work of adhesion. It was reported that PPgMA molecules were chemically bonded on the filler surface due to the acid-base interaction between carboxyl groups grafted on PPgMA and hydroxyl groups from the filler surface. While improvements were noted for PPgMA/MDH blends the surface interactions are fortuitous, different graft copolymers may have to be prepared for every matrix material. Three is no control over the molecular weight of fortuitously grafted to copolymers, hence the morphology of the final composite structure can not be controlled, nor can the amount of material attached to the MDH particle.
Therefore, there is a need for an environmentally benign flame retardant that can be efficiently dispersed in polyolefins, and other thermoplastic and thermoset plastics, to provide both flame retardancy and provide property enhancement. The present invention provides particles with attached polymer chains that are miscible in the matrix polymer. Since they are inherent nanocomposite particles, the attached polymer chains act to provide uniform dispersion of the particles throughout the matrix, thereby improving bulk physical properties such as stress behavior of the resulting alloy.